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Derivatives and Hedging Activities - Additional Information (Details) $ in Thousands | 3 Months Ended
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Dec. 31, 2020 USD ($)
Minimum |
Derivative [Line Items] |
Derivative, maturities | 1 year
Maximum |
Derivative [Line Items] |
Derivative, maturities | 9 years
Weighted average |
Derivative [Line Items] |
Derivative, maturities | 6 years 9 months 18 days
Cash flow hedging |
Derivative [Line Items] |
Notional amount of derivative terminated | $ 100,000
Effective interest rate of derivative terminated | 1.39%

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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
Name: | us-gaap_DerivativeLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Period remaining until the derivative contract matures, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References No definition available. + Details
Name: | us-gaap_DerivativeRemainingMaturity1
Namespace Prefix: | us-gaap_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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- Definition Derivative Terminated, Effective Interest Rate + References No definition available. + Details
Name: | wfsl_DerivativeTerminatedEffectiveInterestRate
Namespace Prefix: | wfsl_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

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- Definition Derivative Terminated, Notional Amount + References No definition available. + Details
Name: | wfsl_DerivativeTerminatedNotionalAmount
Namespace Prefix: | wfsl_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Details
Name: | srt_RangeAxis=srt_MinimumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | srt_RangeAxis=srt_MaximumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | srt_RangeAxis=srt_WeightedAverageMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_DerivativeInstrumentRiskAxis=us-gaap_CashFlowHedgingMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |